Frequency or timing synchronization is the ability to distribute precision frequency around a network. In this context, timing refers to precision frequency, not an accurate time of day. Precision frequency is required in next generation networks for applications such as circuit emulation.

To achieve compliance to ITU specifications for TDM, differential method circuit emulation is used that requires a known, common precision frequency reference at each end of the emulated circuit. The incumbent example of frequency synchronization is provided by SDH equipment. This is used in conjunction with an external timing technology to provide precise synchronization across the network.

SDH equipments are widely replaced by Ethernet equipments and synchronized frequency is required over such Ethernet ports. Synchronized Ethernet (SyncE) is used to accurately synchronize frequency in devices connected by Ethernet in a network. SyncE provides level frequency distribution of known common precision frequency references to a physical layer Ethernet network.

To maintain SyncE links, a set of operational messages are required. These messages ensure that a node is always deriving timing information from the most reliable source and then transfers the timing source quality information to clock the SyncE link. In SDH networks, these are known as Synchronization Status Messages (SSMs). SyncE uses Ethernet Synchronization Message Channel (ESMC) to provide transport for SSMs.

SyncE is supported on Cisco NCS 5001 and Cisco NCS 5002 Routers on 100G, 40G, and 10G interfaces in LAN mode.

Note	Cisco NCS 5001 and NCS 5002 Routers do not support Global Positioning System (GPS). Cisco NCS 5011 Router neither supports SyncE nor GPS.

Source and Selection Points

Frequency Synchronization implementation involves Sources and Selection Points.

A Source inputs frequency signals into a system or transmits them out of a system. There are two types of sources:

• Line interfaces: This includes SyncE interfaces.

• Internal oscillator: This is a free-running internal oscillator chip.

Each source has a Quality Level (QL) associated with it which gives the accuracy of the clock. This QL information is transmitted across the network using ESMC or SSMs contained in the SDH frames. This provides information about the best available source the devices in the system can synchronize to. To define a predefined network synchronization flow and prevent timing loops, you can assign priority values to the sources on each router. The combination of QL information and user-assigned priority levels allow each router to choose a source to synchronize its SyncE or SDH interfaces, as described in the ITU standard G.781.

A Selection Point is any point where a choice is made between several frequency signals and possibly one or many of them are selected. Selection points form a graph representing the flow of timing signals between different cards in a router running Cisco IOS XR software. For example, there can be one or many selection points between different Synchronous Ethernet inputs available on a single line card. This information is forwarded to a selection point on the RSP, to choose between the selected source from each card.

Note	The output of a selection point can be used to drive the signals sent out of a set of interfaces.

To configure Frequency Synchronization on an Interface, use the following procedure:

Note	By default, there is no Frequency Synchronization on a line Interface. Frequency Synchronization must be enabled globally on the router prior to configuring Frequency Synchronization on an interface.

1. Enter the global configuration mode and configure the interface.

   
   RP/0/RP0/CPU0:router# configure
   RP/0/RP0/CPU0:router(config)# interface TenGigE0/0/0/8 

2. Configure Frequency Synchronization on this interface.

   
    RP/0/RP0/CPU0:router(config-if)# frequency synchronization
   

   Note	By default, frequency signal chosen by the system is used for clocking transmission. However, this does not enable the use of the interface as an input.

3. Configure the interface to be used as a timing source by the selection algorithm.

   
   RP/0/RP0/CPU0:router(config-if-freqsync)# selection input

4. Configure priority of the frequency source on a controller or an interface by specifying a value in the range 1 (highest priority) to 254 (lowest priority). The default value is 100. The priority value set is used by the clock-selection algorithm to choose between two sources that have the same quality level (QL) and lower priority values are usually preferred.

   
   RP/0/RP0/CPU0:router(config-if-freqsync)# priority 101

5. Configure the time (in minutes) to wait after the interface is up, before it can be used for frequency synchronization by specifying a value in the range 0 to 12. The default value is 5.

   
   RP/0/RP0/CPU0:router(config-if-freqsync)# wait-to-restore 300

6. Disable the Synchronization Status Messages (SSMs) on the interface to stop sending the packets and ignore the received packets. For SyncE interfaces. sending or receiving ESMC packets is disabled.

   
   RP/0/RP0/CPU0:router(config-if-freqsync)# ssm disable

7. Adjust the QL transmitted by specifying one of the following quality transmit options:

   • exact : Exact QL to be sent. DNU is sent if the exact ql value is not specified.

   • highest : Upper limit on the QL to be sent. QL value specified here is sent, if the QL value of the selected source is higher than the QL value specified here.

   • lowest : Lower limit on the QL to be sent. DNU is sent, if the QL value of the selected source is lesser than the QL value specified here.

   
   RP/0/RP0/CPU0:router(config-if-freqsync)# quality transmit highest itu-t option 1 PRC

8. Adjust the QL received by specifying one of the following quality receive options before the value can be used in the selection algorithm:

   • exact : Exact QL received unless the received value is DNU.

   • highest : Upper limit on the received QL. QL value specified here is used, if the received QL value is higher than the QL value specified here.

   • lowest : Lower limit on the received QL. DNU is used, if the received QL value is lesser than the QL value specified here.

   
   RP/0/RP0/CPU0:router(config-if-freqsync)# quality receive highest itu-t option 1 PRC

   Note	The quality option configured here must match the globally configured quality option specified in the quality itu-t option command.

9. Exit to the global configuration mode and commit your configuration.

   RP/0/RP0/CPU0:router(config-if-freqsync)# commit

Verification

To verify the configured interfaces, use show frequency synchronization interfaces command.

RP/0/RP0/CPU0:router# show frequency synchronization interfaces tenGigE 0/0/0/8

Fri Apr 7 15:06:24.123 UTC
Interface TenGigE0/0/0/8 (down)
  Assigned as input for selection
  Wait-to-restore time 0 minutes
  SSM Enabled
  Input:
    Down
    Last received QL: Failed
    Effective QL: Failed, Priority: 100
    Supports frequency
  Output:
    Selected source: Internal0 [0/RP0/CPU0]
    Selected source QL: Opt-II,1/ST3
    Effective QL: Opt-II,1/ST3
  Next selection points: NCS5K_ETH_RXMUX

To verify the configurations on all line interfaces with frequency synchronization, use show frequency synchronization interfaces brief command.

RP/0/RP0/CPU0:router# show frequency synchronization interfaces brief

Flags: > - Up 												D - Down 												S - Assigned for selection
							d - SSM Disabled 		x - Peer timed out 		i - Init state

Fl 		Interface 														QLrcv 	QLuse 	Pri 	QLsnt 	Source
=== 		====================   ===== 	===== 	=== 	===== 	================
>Sx 	GigabitEthernet0/2/0/0 	Fail 		Fail 		100 	DNU 			None
Dd 		GigabitEthernet0/2/0/1 	n/a 			Fail 		100 	n/a 			None

Sources configured as inputs is indicated by S in the Flags column.

Note	Internal oscillators are always eligible as inputs.

To verify each configured interface separately, use show frequency synchronization interfaces node-id command.

RP/0/RP0/CPU0:router# show frequency synchronization interfaces TenGigE0/0/0/8

Interface TenGigE0/0/0/8 (shutdown)
  Assigned as input for selection
  SSM Enabled
  Input:
    Down
    Last received QL: Failed
    Effective QL: Failed, Priority: 100
  Output:
    Selected source: Internal0 [0/RP0/CPU0]
    Selected source QL: Opt-II,1/ST3
    Effective QL: Opt-II,1/ST3
  Next selection points: NCS5K_ETH_RXMUX